1. Field of the Invention
The present invention relates to a punching unit for punching holes through sheets of paper by engaging punches with dies and more specifically to a punching unit that is suitably mounted to and used in an image forming apparatus such as a copier, printer, facsimile or their combined machine to punch filing holes through sheets of paper.
2. Related Art
The present applicant et al. had developed a punching unit for punching holes through sheets of paper by vertically moving punches by reciprocally moving a cam plate that crosses at right angles with the punch moving direction (see Japanese Patent Application Laid-open No. 2001-9791 for example). As shown in FIG. 6, the punching unit 1 has a body frame 2 that supports a cam plate 3 movably in the horizontal direction and a plurality of punches 6 movably in the vertical direction. The punching unit 1 is also provided with a die plate 7 disposed across a paper passing space S from the body frame 2. Dies (holes) 9 are formed through the die plate 7 at positions facing the punches 6 described above. A plurality of cams 10 respectively formed of a grooved hole is formed through the cam plate 3 at predetermined intervals and an actuation pin 11 is planted to each punch 6, respectively. The body frame 2 is provided with guide holes 12 each formed of a linear long hole extending in the vertical direction to guide the actuation pin 11 in connection with the cam 10.
In operation, revolutions of an electric motor 13 is transmitted through a decelerator 15 to a pinion 16 that is engaged with a rack 17 to reciprocate the cam plate 3. As the cam plate 3 reciprocates, the cam 10 and the guide hole 12 guide the pin 11 and move the punches 6 in the vertical direction. Thus, the punch 6 punches holes through sheets of paper located in the space S with the die hole 9. As the cam plate 3 moves in one direction, the punch 6 upheld while being positioned at one-end linear portion 10a of the cam 10 is reciprocated in the vertical direction at a V-shaped portion 10b of the cam 10 and is then upheld again by moving to another-end linear portion 10c. At this time, the pin 11 is guided by the guide hole 12 formed of the linear long hole, so that the punch 6 moves in the vertical direction without turning around its axial line.
In a state in which a capacity of the driving motor 13 is limited to a predetermined capacity and a thrust force Q of the cam plate 3 is limited due to an installation space and others of the punching unit 1, it is effective to reduce an inclined angle (wedge angle) α of the V-shaped portion 10 b of the cam 10 in order to obtain a large punching force F of the punch 6.
Because it is necessary to assure a punch moving distance PL to punch holes through sheets of paper, a moving distance (slide stroke) SL of the cam plate 3 must be increased if the inclined angle α of the cam 10 is to be reduced. If the moving distance SL of the cam plate 3 is thus increased, a time required for punching holes is prolonged and the size of the punching unit becomes large, so that it is unable to deal with the high speed operation of the late image forming apparatus such as a printer and to punch through heavy papers. Thus, it is desired to bring out a high-performance punching unit that takes an equivalent installation space with that of the prior art punching unit and does not involve a large modification.